DNA methylation is a post-replication modification which confers protection or specificity to restriction endonucleases in bacteria, but a unique role in cells of higher organisms has not been discovered. Nevertheless, the modification is widespread, if not universal, in both animal and plant cells. There are two possible roles that could make it important in aging. One is that it confers specificity for which chain is to be repaired when mistakes are made in base pairing in replicating DNA. Before modification the newly replicated, unmodified chain might be preferentially excised and repaired. The second is that the pattern of methylation is a regulatory signal in the functioning of a genetic locus which can be modified during differentiation and development and thereafter propagated enzymatically each cell division by the information in the symmetrically modified parental chains. Either of these roles could make the process of methylation a significant factor in aging at the cellular level by changes or losses of the pattern or sequence specificity. For example, if two replications occur without the modification, any information contained in the symmetrically modified parental chains will be lost from two of the resulting four double helices of DNA. To assess the possible significance of the modification, a study of the sequences methylated and the pattern of conservation or change during developmental stages and over the cell cycle should be done with the best tools available to molecular biologists. These include the restriction endonucleases, nearest neighbor analysis of methylated cytosines, and the production of DNA with modified or deleted methylation patterns to see their effects during replication and other cellular functions.